The present invention relates to methods of controlling the density, permeability, moisture retention and thermal properties of bulk materials and to compositions produced by the methods.
Efficient, low cost transportation and storage of bulk materials from mines and/or factories to markets are vital to certain industries because the costs of transporting and storing bulk material are often major components of the total cost of the delivered product.
Coal is one of the world""s largest bulk commodities moved by rail, truck, inland barges and ocean-going vessels to utilities and steel mills. The cost of transporting coal plays a critical role in expanding markets for coal. Changes in environmental laws in the United States have created a demand for low-sulfur, premium quality steam coal. Before 1975, underground mines in West Virginia and eastern Kentucky supplied most of the premium quality coal needed to meet environmental requirements at coal-fired utilities. Although vast low-cost, strippable reserves of low sulfur coal resided in the West, distance and associated high transportation costs excluded them from serious consideration for Midwestern and Eastern markets. This situation changed as railroads recognized the opportunity for new markets and began investing in unit trains and improved ways and structures to haul large tonnage shipments. As a result, large productive mines were developed in the Powder River Basin (PRB) in Wyoming. Production of PRB coal has risen steadily since 1980 replacing higher cost eastern coal. Production is expected to rise to 400 million tons per year in the near future. Since transportation can account for up to 75% of the total delivered price, it continues to play the critical role in expanding the market for western coal. The increased demand for PRB and other western coal will not be realized unless the railroad companies continue to find ways to reduce costs and improve efficiency.
The market for metallurgical coal is also dependent on the cost of transportation. For example, steel mills are extremely competitive and are constantly looking for lower cost coal to fuel their blast furnaces. Although the best quality metallurgical coal in the world reside in the eastern United States, Australian and South African producers often win contracts because of lower costs. The high cost of transporting coal by rail from eastern mines to port facilities often makes American suppliers non-competitive.
Coal has a low bulk density compared to many other common bulk materials, such as limestone, aggregates, iron ore and fertilizers. Since coal is hauled in the same rail cars, trucks, barges and ocean-going vessels as the more dense bulk materials, less weight can be carried for a given volume of cargo hold. The full weight carrying capacity of many vessels cannot be reached before the volumetric capacity is reached. As a result, costs are increased since the weight capacity of the vessel is underutilized. Consequently, a coal producer is penalized because a rail car cannot be loaded to full weight carrying capacity. One PRB mine operator reported that underweight penalties cost about $100,000 per month, totaling over $1 million ina recent year.
Storage and handling costs are also affected by bulk density similar to transportation costs. As bulk density increases, less storage volume is required to hold the same amount of coal. Smaller stockpiles require less area to hold coal resulting in lower storage costs. Likewise, the smaller volume also requires less loading and unloading time and labor.
When bulk materials are hauled in conveyances such as rail car, barges, and trucks during cold weather, moisture contained in the material may form ice that can adhere to the conveyance. Frozen material, accounting for up to 10 percent of the net payload, may not discharge from the conveyance at the point of delivery. The added weight increases transportation costs by reducing the useful carrying capacity of the conveyance and increasing the weight of the conveyance returned to the producer.
Sub-zero temperatures and long transit times can cause the payload to freeze creating large lumps of aggregated material, particularly when water goes through the material and pools at the bottom of the conveyance before freezing. As a result, special equipment is required to break the frozen lumps into manageable sizes that are compatible with material handling and storage equipment.
Two principal methods are typically used to mitigate the adverse effects of frozen material. The first method involves adding a chemical such as a salt compound or liquid glycol antifreeze to the bulk material to depress the freezing point of water or weaken the ice that binds the solid particles together as described, for example, in U.S. Pat. No. 5,079,036 entitled xe2x80x9cMethod of Inhibiting Freezing and Improving Flow and Handleability Characteristics of Solid, Particulate Materialsxe2x80x9d and in U.S. Pat. No. 4,290,810 entitled xe2x80x9cMethod for Facilitating Transportation of Particulate on a Conveyor Belt in a Cold Environment.xe2x80x9d The second principal method involves heating the walls of the conveyance to thaw the frozen layer of material adhering to the walls as described, for example, in U.S. Pat. No. 4,585,178 entitled xe2x80x9cCoal Car Thawing Systemxe2x80x9d and in U.S. Pat. No. 4,221,521 entitled xe2x80x9cApparatus for Loosening Frozen Coal in Hopper Cars.xe2x80x9d Several manufacturers offer electric and gas-fired radiant heaters to warm the bottom and sides of a conveyance to melt the frozen layer of material. The choices of chemical or thermal methods depend on the type of conveyance, cost constraints, and material compatibility. Treating frozen materials has become more expensive because many rail cars are fabricated from aluminum, a thermally sensitive material that can corrode when it comes in contact with low-cost salt compounds.
Thawing and chemical treatment methods are time consuming and expensive. Thawing costs range between $0.20 and $0.50 per ton, depending on the source of energy. Chemical treatment costs range between $0.20 and $1.00 per ton, depending on the type of chemical and dose rate.
Most bulk materials that are crushed to a specified topsize for commercial reasons have a naturally occurring particle size distribution that, when plotted, fit under a typical single gaussian curve. Such naturally occurring size distribution does not have the optimum particle size distribution to produce sufficiently high bulk densities to effectively lower transportation and storage costs or to mitigate the effects of freezing. In addition, known methods of altering the thermal properties of bulk materials, such as lowering permeability and increasing moisture retention, result in decreasing the bulk density since the materials are simply crushed into a smaller size in an attempt to increase the surface area of the bulk material.
Compacting or vibrating is commonly used to increase bulk densities by many industrial applications that handle relatively small volumes of high-value fine powders (0.5 mm and smaller). Examples include pharmaceuticals, cosmetics, ceramics, sintered metals, plastic fillers and nuclear fuel elements. However, many applications that involve large volumes of coarse bulk materials (up to 150 mm) cannot effectively use compaction or vibration to control bulk density. If the coarse material is of relatively high value, expensive oil or other chemical additives that modify the particle surface characteristics can be applied to modify bulk density. For example, steel mills typically control bulk density of metallurgical coal feeding cooking ovens by applying additives as described in U.S. Pat. No. 4,957,596 entitled xe2x80x9cProcess for Producing Coke.xe2x80x9d
Accordingly, a need exists for low cost methods of controlling the density, permeability and moisture retention of bulk materials. The present invention satisfies this need and provides related advantages.
The present invention relates to methods of controlling the density, permeability, moisture retention and thermal properties of bulk materials and to compositions produced by such methods. Bulk materials that can be controlled by the present methods include any material that can be fractionated by particle size and include, for example, solid fuel materials, limestone, bulk food products, sulfide ores, carbon-containing materials such as activated carbon and carbon black. Solid fuel materials include, for example, coal, lignite, upgraded coal products, oil shale, solid biomass materials, refuse derived fuels (including municipal and reclaimed refuse), coke, char, petroleum coke, gilsonite, distillation byproducts, wood byproducts, shredded tires, peat and waste pond coal fines.
In one embodiment, the present invention relates to methods of increasing the density of a bulk material by combining two different particle sized fractions to form resulting bulk material having a bimodal size distribution. The methods are generally accomplished by first separating the bulk material into a first size fraction and a smaller size fraction. The smaller size fraction is next separated into a second size fraction and a third size fraction, in which the second size fraction is larger than the third size fraction. The second size fraction is then sized into a fourth size fraction, which is the same size as the third size fraction. The final step is combining the first size fraction with the third and fourth size fractions to produce a densified bulk material. Optionally, the methods can also include a step of sizing the starting bulk material into a desired topsize before separating out a first size fraction.
For example, in one embodiment the method is accomplished by recovering a first size fraction of the bulk material having a particle size of about 1 inch to about 2 inches, followed by recovering a third fraction of the bulk material having a particle size of less than about xc2xc inch from a second size fraction having a particle size of about xc2xc inch to about 1 inch and subsequently crushing, grinding or pulverizing the second size fraction to form a mixture having a particle size of less than about xc2xc inch. In the final step, the first, crushed second and third size fractions are combined to produce a higher density bulk material. Mixing these fractions provides the fine particles an opportunity to occupy the void between the coarse particles to achieve the highest bulk density. Accordingly, the present invention is based, in part, on the discovery that mid-sized particles impede the flow of the fine particles in filling this void, which results in lower bulk density.
In alternative methods for increasing the density of a bulk material, the bulk material is first fractionated into increasingly smaller particle fractions. The largest particle size fraction is placed into a holding area or compartment. The next smaller fraction is then added to fill the void between the larger particles. Filling is continued until the smaller particles begin to dilate the entire mixture (i.e., push the larger particles apart) thus reducing bulk density. At that point, the next smaller size fraction is added filling the void until the entire mixture begins to dilate. This process is continued with each successive smaller size fraction. Although the methods of this embodiment may require more processing steps than the first embodiment, it can be used to obtain a higher density and, therefore, may be preferred for certain applications.
The methods of increasing the density of bulk materials result in bulk materials having a density of at least 55 lbs/ft3, with a useful range between about 55 lbs/ft3 to about 60 lbs/ft3.
Methods for improving thermal properties of bulk materials without reducing density are also provided. The methods are generally accomplished by first separating the bulk material into a first size fraction and a smaller size fraction. The smaller size fraction is next separated into a second size fraction and a third size fraction, in which the second size fraction is larger than the third size fraction. The second size fraction is then sized into a fourth size fraction, which is the same size as the third size fraction. The final step is combining the first size fraction with the third and fourth size fractions to produce a final bulk material having improved thermal properties, such as reduced permeability and increased moisture retention. Optionally, the methods can also include a step of sizing the starting bulk material into a desired topsize before separating out a first size fraction.
Preferably, the permeability of the final bulk material is reduced at least about 50%, more preferably is reduced at least about 90%, while the moisture retention capacity is increased at least about 25%, more preferably at least about 50%. For coal, the permeability is preferably less than about 0.040 cm/sec, more preferably less than about 0.020 cm/sec, and most preferably less than about 0.004 cm/sec. In a further embodiment, the present invention also provides methods for reducing the density of bulk materials. The density of bulk materials can be reduced by creating more void space between particles to promote, for example, flow of gases and liquids between particles. Accordingly, such methods would be useful for storing or treating bulk materials with chemicals or when exposure to heat, air (i.e. oxidation), other gases or liquids is desired.